The function new_fieldtype creates a new field type usable
for data validation. You supply it with field_check, a
predicate to check the validity of an entered data string
whenever the user attempts to leave a field. The (FIELD
*) argument is passed in so the validation predicate can
see the field's buffer, sizes and other attributes; the
second argument is an argument-block structure, about
which more below.
You also supply new_fieldtype with char_check, a function
to validate input characters as they are entered; it will
be passed the character to be checked and a pointer to an
argument-block structure.
The function free_fieldtype frees the space allocated for
a given validation type.
The function set_fieldtype_arg associates three storage-
management functions with a field type. The make_arg
function is automatically applied to the list of arguments
you give set_field_type when attaching validation to a
field; its job is to bundle these into an allocated argu-
ment-block object which can later be passed to validation
predicated. The other two hook arguments should copy and
free argument-block structures. They will be used by the
forms-driver code. You must supply the make_arg function,
the other two are optional, you may supply NULL for them.
In this case it is assumed that make_arg does not allocate
memory but simply loads the argument into a single scalar
value.
The function link_fieldtype creates a new field type from
the two given types. They are connected by an logical
'OR'.
The form driver requests REQ_NEXT_CHOICE and
REQ_PREV_CHOICE assume that the possible values of a field
form an ordered set, and provide the forms user with a way
to move through the set. The set_fieldtype_choice func-
tion allows forms programmers to define successor and pre-
decessor functions for the field type. These functions
take the field pointer and an argument-block structure as
arguments.

The pointer-valued routines return NULL on error. They
set errno according to their success:
E_OK The routine succeeded.
E_BAD_ARGUMENT
Routine detected an incorrect or out-of-range argu-
ment.
E_SYSTEM_ERROR
System error occurred, e.g., malloc failure.
The integer-valued routines return one of the following
codes on error:
E_OK The routine succeeded.
E_BAD_ARGUMENT
Routine detected an incorrect or out-of-range argu-
ment.
E_CONNECTED
The field is already connected to a form.
E_CURRENT
The field is the current field.
E_SYSTEM_ERROR
System error occurred (see errno).

The header file <form.h> automatically includes the header
file <curses.h>.
All of the (char*) arguments of these functions should
actually be (void*). The type has been left uncorrected
for strict compatibility with System V.